1. Technical Field
Embodiments of the present invention relate to a frequency locked loop, more particularly to a frequency locked loop of a heterodyne structure for locking the frequency of a voltage-controlled oscillator.
2. Description of the Related Art
High frequency bands can be utilized in various fields, such as imaging, spectroscopy, biochemical detection, astronomy, and broadband communication. There is growing interest in such frequency bands, and researchers are motivated to implement systems based on semiconductor elements due to the advantages of small size, low cost, low power consumption, etc.
An imaging receiver terminal for a high frequency band may be composed of a direct detection structure and a heterodyne structure. The direct detection structure has the advantages of consuming less power and occupying a small area. The heterodyne structure consumes more power, occupies a larger area, and entails a more complicated design compared to the direct detection structure but provides the advantage of superior performance. FIG. 1 illustrates the structure of a conventional heterodyne receiver.
To increase the detection speed of an image, an array receiver terminal with a high resolution is required. If a heterodyne receiver structure is used for the array receiver terminal, the phase noise of the LO (local oscillator) signal for each pixel has to be low. Otherwise, as an error may occur with each instance of output values for the pixels, the resulting noise may become large. Methods for decreasing the phase noise of LO signals include methods that use the phase locked loop and the frequency multiplier chain.
Since the area per pixel of the array receiver terminal decreases with an increase in the frequency band, the receiver terminal has to be made small so as to be integrated in the area of each pixel. When an array receiver terminal is implemented in a heterodyne structure, there is a difficulty in integrating each heterodyne receiver in each pixel, as using a phase locked loop or a frequency multiplier chain occupies additional area.